Device for sampling and holding the carrier frequency of short duration pulses

ABSTRACT

A device for measuring the frequency of a pulsed carrier signal in which the carrier frequency is situated within the frequency range and to give a continuous signal whose frequency is equal to the measured carrier frequency. A frequency discriminator (3) gives an analog voltage via a track-and-hold circuit (4) and a DC-amplifier (15) to one input of a comparator (6) whose second input receives the analog voltage from the discriminator (3). The difference value from the comparator (6) is supplied to a voltage controlled oscillator (8). This is via a directional coupler or duplexer (9) and a controllable switch (1) connected to the frequency discriminator (3) which alternatively receives the input signal to the device and the output signal from the voltage controlled oscillator (8).

TECHNICAL FIELD

The present invention relates to a device for sampling and regeneratingthe carrier frequency of each of the individual pulses in an incomingpulsed carrier frequency signal. The device according to the inventioncan, for example, be included as a part in a radar receiver, the carrierfrequency being situated within the microwave range.

BACKGROUND ART

In, for example, the radar art it is frequently necessary to sample thefrequency of the individual pulses in an incoming pulse train to thereceiver. This can cause problems, especially, when every incoming pulsehas a very short duration and particularly when the carrier frequencyvaries from one pulse to another. The incoming signal consists, forexample, of pulses having mutually different frequency values, which forexample, constitutes the signal from different radar stations. Thedevice should after the sampling deliver a signal with the samefrequency as the received carrier. Examples of known circuits or systemsfor this purpuse is described in, for example, the U.S. Pat. No.3,996,588.

DISCLOSURE OF INVENTION

A general object of to the present invention is to solve such problem.During the duration of a pulse the value of the carrier frequency of thepulse is measured by means of a frequency discriminator, the thusmeasured value is then supplied to a voltage controlled oscillator as acontrol quantity. This control quantity is allowed to control theoscillator to a frequency approximately equal to or in the neighbourhoodof the received frequency during the short time interval of the carrierfrequency pulse duration. During the subsequent longer time interval (orpart thereof), when no carrier frequency pulse is received the deviceworks as a closed loop and the voltage controlled oscillator iscontrolled by the value of a difference signal and is adjusted to afrequency value near the value of the carrier frequency pulse detectedduring the previous time interval.

The novel and the characterizing of the present device is atrack-and-hold circuit and an amplifier connected between the frequencydiscriminator and the voltage controlled oscillator. A continuous signalwhich follows the output signal from the discriminator is delivered onthe one hand directly and on the other hand via a track-and-hold circuitand the amplifier to a difference forming circuit and during the timeinterval of the pulse duration. The output signal of the differencecircuit controls the oscillator. During the subsequent longer timeinterval the last measured value from the discriminator is stored in thetrack-and-hold circuit and the oscillator is controlled in dependence onthe difference between a value corresponding to the oscillator frequencyand the stored value.

Accordingly, a continuous control of the voltage controlled oscillatoris attained already from the beginning of a carrier frequency pulse,i.e. sudden changes in the oscillator control is avoided which implies amore rapid and reliable measuring and adjusting of the carrierfrequency.

A specific object of the present invention is thus to provide a deviceto rapidly and securely sample and regenerate the carrier frequency ofreceived pulses in a pulse train where the carrier frequency can varyfrom one pulse to another and to give an accurate determined analogcounterpart to every detected and measuring frequency, for example inthe form of a direct voltage.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be closer described with reference to theaccompanying drawings in which:

FIG. 1 shows a block diagram of the device according to the presentinvention;

FIG. 2 shows a timing diagram in order to explain the function of thetrack-and-hold circuit which is included in the dvice according to theinvention;

FIG. 3 shows a block diagram equivalent with FIG. 1 during the timeinterval the device forms an open control loop; and

FIG. 4 shows a circuit diagram equivalent with FIG. 1 during the timeinterval the device forms a closed loop.

MODE FOR CARRYING OUT THE INVENTION

The block diagram according to FIG. 1 shows a device according to theinvention. The block 1 designates a du path controlled switch having oneinput 1a receiving the signal U0. Signal U0 is shown in FIG. 2consisting of a pulsed carrier frequency signal whose frequency f₁, f₂ .. . from one pulse to another is to be measured. When the device is usedin, for example, a radar receiver the frequencies f₁, f₂ are situatedwithin the microwave range. The other input 1b of the switch 1 isconnected to a duplexer 9. The output 1c is connected to a frequencydiscriminator 3 via an amplifier-limiting stage 2. Across the output ofthe discriminator 3 an analog signal quantity U3 is obtained whose valuedepends on the frequency value of the incoming microwave signal U2 fromthe amplifier stage 2. The discriminator 3 is assumed to be linear, i.e.the value of the analog signal U3 is directly proportional to thedetected frequency with a proportion factor Kd (V/MHz). The output ofthe discriminator 3 is connected on the one hand to the input of atrack-and-hold circuit 4 and an the other hand to the minus input of anoperational amplifier 6. THe track-and-hold circuit 4 is with its outputconnected via a DC-amplifier 5, with the amplification K₁, to the pulseinput of the operational amplifier 6, the output signal (DC-voltage)from the amplifier 5 being designated U₅, and forms the amplified valueof the signal U₄. The operational amplifier 6 then constitutes adifference forming circuit or a comparator, which compares thequantities U₃ and U₅. A difference signal is formed which is fed via asecond DC-amplifier 7 with the amplification K2 to a voltage controlledoscillator (VCO) 8. This is assumed, like the discriminator 3, to belinear and is characterized by the factor K_(V) (MHz/V), a signal U₈being delivered whose frequency is situated within the microwave rangeand the value being dependent of the value of the analog quantity U₇from the amplifier 7.

The signal U₈ is delivered via the duplexer 9 on the one hand to thesecond input of the switch 1 and on the other hand to the output of thedevice which can be connected to a utilization device (not shown). Inthe diagram according to FIG. 1 thus microwave signals appear to theleft of the dashed line a--a and analog signals in the form ofDC-voltages to the right of the line a--a.

Before describing the operation of the circuit, briefly the function ofthe track-and-hold circuit 4 will be discussed with reference to thetiming diagram according to FIG. 2. At the time t₀ and during the timeinterval t₀ -t₁ the control signal Tlsw to the switch 1 is such that thecontacts are in the position 1a-1c, signal U1 being equal to signal U0.The control signal Ts is then such that the track-and-hold circuit 4 isfully conducting, i.e. signal U4 is equal to signal U3. The input signalU0 consists, for example, of a pulsed carrier frequency signal as shownin FIG. 2 having a low value duty cycle of the magnitude 1/1000. Theinput signal U2 to the frequency discriminator thus consists of a pulsetrain with short duration pulses, where the carrier frequency can varyfrom one pulse to another. The leading edge of the pulse coincidesmainly with the instant t₀ while its trailing edge appears at theinstant t₁ or at some instant later. During the time t₀ -t₁ the carrierfrequency is measured by the discriminator 3, the output signal U3consisting of an analog signal whose value is proportional to thefrequency of the pulse. The signal U3 is after that conducted unchangedthrough the closed track-and-hold circuit 4.

At the time t₁ a control pulse Ts is delivered to the track-and-holdcircuit which implies the value of the signal U₃ at the time t₁ "isfrozen" (stored), so that the output signal U₄ consists of an analoguesignal with a constant value during the interval t₁ -t' (equal to thevalue U3 at the time t₁). At the same time, t₁, the switch 1 is changedby the signal Tlsw to the position 1b-1c. At the time t₂ thetrack-and-hold circuit is switched so that the signal U3 is completelylet through (as at the time t₀) and the process is repeated. The circuit4 can as is known consist of a series-parallel-circuit of a switch (forexample a transistor) in the series path and a capacitor in the parallelpath.

When the switch is closed, the capacitor is charged to the value of thesignal U3 (the interval t₀ -t₁) and when the switch is disconnected (theinterval t₁ -t') the capacitor charge (-voltage) constitutes the holdingvalue. The circuit 4 thus "tracks" the input signal during the timeinterval t₀ -t₁, t₂ -t₃ and "holds" (stores) during the interval t₁ -t'the value of the input signal which last appeared.

The operation of the frequency sampling circuit will now be more fullydescribed with reference to FIGS. 3 and 4 where FIG. 3 shows the circuitduring the time interval t₀ -t₁, t₂ -t₃ (open loop) and FIG. 4 shows thecircuit during the interval t₁ -t' (closed loop). The discriminator 3and the voltage controlled oscillator 8 are assumed to be linear withthe factors Kd (V/MHz) and Kv (MHz/V), respectively. Each of theamplifiers 5 and 7 is assumed to have a linear characteristic with theamplification factor K₁ and K₂, respectively. Then following is valid:

    U4=U3=Kd×fi (fi is the frequency of the carrier wave pulse)

    U5=K.sub.1 ×Kd×fi

    U6=Kd×fi×(K.sub.1 -1); K.sub.1 ≠1,

the frequency fv of the output signal U8 from the voltage controlledoscillator 8 being: fv=Kx(K₁ -1)×fi . . . (1) if Kv×K₂ ×Kd=K. Duringthis time interval a signal U3 is fed to the track-and-hold circuit 4which varies according to the frequency of the input signal and thus alikewise varying input signal U7 to the oscillator 8. The frequency fvthus varies but is proportional to the frequency fi of the input signalU1.

FIG. 4 shows the circuit during the time interval t₁ -t'. The loop isnow closed and the track-and-hold circuit 4 delivers a constant signalto the amplifier 5, which is fed to the plus input of the amplifier 6.Across the minus input a signal U3 is fed, which is proportional to thefrequency fx of the voltage controlled oscillator. The followingrelations can be set:

    fx=Kv×K.sub.2 ×(K.sub.1 ×Kd×fi-Kd×fx)=K×K.sub.1 ×fi-K×fx

which gives fx=[(K×K₁)/(1+K)]×fi.

If K₁ =1+1/K, it is obtained

    fx=fi

This implies that the frequency of the voltage controlled oscillator hasbeen controlled to exactly the same value as the received carrierfrequency.

Putting the value K₁ =1+1/K into the equation (1) it is obtained:

    fv=fi

i.e. also the frequency of the output signal from the oscillator 8during the interval t₀ -t₁ is exactly the same as the received carrierfrequency.

By means of the described device an oscillator can be held to thecarrier frequency of a sampled short duration pulse from the beginningof the sample of the pulse and controlled to the sampled value after thepulse and that this is carried out without any control error and that atthe output of the amplifier 5 the signal K₁ ×K_(d) ×fi which thusconstitutes an accurate analog signal proportional to the incomingcarrier frequency.

What we claim is:
 1. A carrier frequency sample and hold circuitcomprising switching means controllably switching between a first inputadapted to receive a carrier frequency pulse signal of low duty cycle, asecond input and an output, frequency discriminator means for convertingthe carrier frequency signal to an analog signal having an amplitudewhich is a function of the frequency of an input signal, said frequencydiscriminator having an input connected to the output of said switchingmeans and an output, an analog track and hold means periodicallyswitchable between a track and hold mode in synchronism with saidswitching means, said analog track and hold means having an inputconnected to the output of said frequency discriminator means and anoutput, a difference amplifier means having first and second inputsconnected respectively to the outputs of said frequency discriminatormeans and said track and hold means and an output, voltage controlledoscillator means for emitting a signal having a frequency related to theamplitude of a received signal, said voltage controlled oscillator meanshaving an input connected to the output of said difference amplifiermeans and an output connected to the other input of said switchingmeans, and utilization means connected to the output of said voltagecontrolled oscillator means.